1. Field of the Invention
This invention relates to a rubber-reinforcing steel wire for use in a steel radial tire, a high-pressure hose, or a conveyer belt, and a method for manufacturing the same.
2. Description of the Related Art
In a steel wire to be used as a rubber-reinforcing member in a steel radial tire, a high-pressure hose, or a conveyer belt, the adhesive properties between the wire and vulcanized rubber and the persistence of the properties are important factors. A conventional practical rubber-reinforcing steel wire generally has a coating layer made of brass for enhancing the adhesive properties between the wire and rubber. The brass layer can be formed by alloy plating using a cyanide bath. In this method, however, pollution arising from use of cyanide must be avoided, and it is relatively difficult to control Cu content of the brass. Thus, diffusion plating, in which a thermodiffusion treatment is performed after Cu and Zn are plated in order, has become a technique used widely. As heating methods, there is employed electric heating, high-frequency heating, heating using a fluidized bed furnace, etc.
During vulcanization, Cu contained in brass exhibits a high reactivity against sulfur contained in rubber, whereas Zn properly reduces Cu reactivity. As a result, a proper quantity of sulfide can be formed at the interface between the coating layer of the wire and rubber. It is believed that good primary adhesive property to rubber is obtained by such a mechanism. However, a rubber-reinforcing steel wire having a brass-plating layer is disadvantageous in that the adhesive property to rubber under high humidity conditions i.e. wet adhesive property is degraded. To improve this property, it is considered preferable to prevent excessive generation of sulfide at the interface between the coating layer and rubber, and to minimize the Cu content of the brass-plating layer.
Further, to enhance the strength of the steel wire and the rubber reinforcing effect, the wire is subjected to cold working, especially drawing using a die. The drawing is performed under very severe conditions. Therefore, if the brass plating layer has low deformability, a great amount of friction heat will be generated, the wire will have high resistance against drawing, the life of the die will be reduced, or the wire may even be broken during drawing.
As regards the brass plating layer, the a phase crystal has a higher ductility and malleability than the .beta. phase crystal. This is because the .beta. phase has a body-centered cubic lattice, whereas the a phase has a face-centered cubic lattice and hence has lots of slip planes. To enhance the drawability, the ratio of the .beta. phase to the a phase in the brass-plating layer is preferably low. If diffusion plating is used, the crystal phase can be influenced by the heating conditions. It is known that the higher the heating temperature and the longer the heating time, the more partial diffusion is likely to occur and the lower the ratio of the .beta. phase. In view of this, optimal conditions are selected.
However, if Cu content of the brass plating layer is reduced so as to improve the wet adhesive property, it is known that the ratio of the .beta. phase increases. Where the Cu content of brass is relatively low, e.g. lower than 62%, it is difficult to perform drawing. From the industrial point of view, the lower limit of the Cu content of brass is about 63%. In summary, therefore, it is very difficult to obtain both good wet adhesive property and good drawability.
With the aim of overcoming the above drawback, a technique has been proposed for improving the wet adhesive property by forming a coating layer made of an alloy consisting of brass and a third element (X). As the third element (X), there is used Ni (disclosed in Published Unexamined Japanese Patent Application No. 55-105548), Co (Published Examined Japanese Patent Application No. 1-37411), Fe (Published Examined Japanese Patent Application No. 2-39599), or the like. In these disclosures, a coating layer is formed by laminating Cu, X, and Zn plating layers in this order, and is then subjected to a thermodiffusion treatment. The reason the X plating layer is interposed between the Cu and Zn plating layers is that it is difficult to tightly laminate the Cu plating layer on the Zn plating layer. However, when the third element (X) has a high melting point, the X plating layer functions as barrier layer which hinders partial diffusion of Cu and Zn, thereby increasing in the ratio of the .beta. phase and degrading the drawability of the steel wire.
A method of performing a thermodiffusion treatment after laminating Cu, Zn, and X plating layers in this order also has been proposed. In this structure, the plating layer of the third element does not hinder partial diffusion of Cu and zn. However, a Zn-X phase is created, and the drawability of the steel wire will be significantly degraded depending on the ductility of this phase. Moreover, a method of performing a thermodiffusion treatment after laminating X, Cu, and Zn plating layers in this order has been proposed. In this structure, the plating layer of the third element merely functions as an underlying layer, with the result that a desired alloy of the third element and brass plating cannot be obtained, therefore, a satisfactory effect of the third element cannot be realized.
Yet another, a method of alloy plating a steel wire with Cu-Ni, where Ni is used as a third element, plating it with Zn and then performing a thermodiffusion treatment, has been proposed. In this method, it is necessary, but quite difficult, to control the Ni content of the alloy in accordance with the Cu-Ni plating conditions. Further, the .beta. phase is likely to grow under the influence of Ni having a high melting point. Moreover, the Ni will be diluted as a result of partial diffusion with Cu-Zn, and hence the addition amount of Ni must be increased so as to improve the wet adhesive property.
Published Examined Japanese Patent Application No. 52-14778 discloses a method of alloy plating a steel wire with Cu-Zn (brass), plating it with Sn used as a third element and then drawing it. However, the steel wire obtained by this method is advantageous only in enhancing the adhesive property of the wire to rubber containing moisture during vulcanization, and also in reducing the possibility of breakage of a steel cord due to heat or wear caused during use of a tire. Accordingly, an improved secondary wet adhesive property is not realized.